1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device.
2. Description of the Related Art
An exposure apparatus (stepper) of the step-and-repeat scheme transfers a pattern on a reticle onto a substrate by reducing light passing through the reticle at a predetermined ratio using a projection optical system and exposing a photoresist applied on the substrate to light while keeping the substrate in a stationary state in a predetermined place. This apparatus repeatedly performs shooting operation on the entire surface of the substrate by sequentially driving a substrate stage on which the substrate is placed.
As compared with a stepper designed to perform one-shot exposure on a stationary substrate, an exposure apparatus (scanner) of the step-and-scan scheme exposes a wider area to light while synchronously scanning a substrate and a reticle.
In general, in the semiconductor manufacture using an exposure apparatus, it is necessary to repeatedly perform exposure on the same substrate. That is, it is necessary to perform exposure on a shot on which a pattern has already been formed upon superimposing another pattern on it with high accuracy. For this purpose, it is necessary to measure an array of shots in advance. This measurement uses a technique of recording measurement marks on patterns in advance and measuring the marks by using a scope. As this scope, an off-axis scope (to be abbreviated as an “OAS” hereinafter) using non-exposure light is generally used. Although the OAS can perform measurement without exposing a resist to light because it performs measurement by using non-exposure light, since light cannot be made to pass through a lens, the OAS and the lens need to be placed apart from each other, as shown in FIG. 6.
In order to measure a mark with the OAS, it is necessary for the OAS to measure the mark at the best focus. There is also available a technique of searching for a position at which the image plane of the scope of an OAS is located at the best focus by driving a substrate stage in the Z direction. This technique, however, takes a lot of time to perform measurement, and hence is disadvantageous in terms of throughput. In general, therefore, a technique using another focus sensor is used.
The focus sensor placed at the OAS position is used to align a substrate surface with the best focus position of the OAS, but is not used for actual exposure. In general, when performing exposure below a lens, a scanner performs real-time focusing to focus the lens while driving the substrate stage. As shown in FIG. 6, the focus sensor is placed at a position shifted from the actual exposure position (slit) backward/forward, that is, in the Y direction. When performing exposure upon scanning/moving the substrate stage from a lower position on the drawing surface, the apparatus measures the Z position of a substrate surface at a measurement point 6C with the focus sensor before exposure, and drives the substrate stage in the Z direction before an exposure position reaches the slit position. When scanning/moving the substrate stage from an upper position on the drawing surface, the apparatus uses a measurement point 6A of the focus sensor in the same manner. In order to check whether the substrate surface is driven into the lens image plane below the slit, a measurement point 6B of the focus sensor is also placed at the slit position. In addition, arranging a plurality of focus sensor measurement points in the X direction can also detect the tilt component of the substrate surface within the slit. As described above, since a focus is measured immediately before exposure, there is no need to measure the focus of the entire substrate surface in advance. That is, there is no reduction in throughput.
The following is an exposure sequence using the OAS. When a substrate is transferred, the apparatus measures the mark of each sample shot on the substrate by using the OAS. The apparatus calculates error components such as substrate placement errors (XY and rotation components) and thermal expansion from the measurement result. The apparatus then exposes each shot to light by moving the substrate to a position below the lens. As the number of sample shots increases, the superimposition accuracy increases, but the throughput decreases.
As a scheme of improving throughput and accuracy by simultaneously performing measurement and exposure on a sample shot, an exposure apparatus with a twin-stage arrangement is available, which includes two substrate stages and performs exposure on one substrate while performing measurement on the other substrate by using an OAS. Note that an arrangement having one substrate stage is called a single-stage arrangement.
FIG. 7 shows a twin-stage arrangement. When a substrate is placed on one substrate stage, this apparatus measures the displacement of a shot in the X and Y directions first using the OAS. The apparatus then performs measurement on the entire substrate surface at measurement points 7B using a focus sensor. The apparatus performs this operation concurrently with exposure on a substrate on the other substrate stage. If the apparatus completes focus measurement by performing measurement on one substrate by using OAS before exposure on the other substrate, even increasing the number of sample shots will not degrade the throughput. It is therefore possible to satisfy both the requirements for throughput and accuracy.
In general, the measurement points 7B of the focus sensor are arrayed in the X direction. This arrangement shortens the measurement time by measuring more widths when scanning in the Y direction. Laser interferometers are respectively arranged at a lens position and an OAS position.
A substrate stage is required to be driven to an arbitrary place on a two-dimensional plane (X-Y plane) with high accuracy throughout a wide range. This is because, the accuracy required becomes severer with a reduction in the size of a semiconductor circuit, and at the same time, the driving range of the substrate stage needs to be a very wide to cope with an increase in the diameter of a substrate.
A laser interferometer is generally used to detect the position of a substrate stage. Placing the laser interferometer within an X-Y plane can measure the position of the substrate stage within the X-Y plane. For example, as shown in FIG. 1, a bar mirror 2A for X-axis measurement is mounted on the substrate stage 1 along the Y-axis direction. A laser interferometer 3A1 to measure a position in the X-axis direction detects the relative driving amount of the substrate stage by irradiating the bar mirror 2A with laser light almost parallel to the X-axis and making the reflected light interfere with reference light. The same applies to measurement on the Y-axis. Preparing two interferometers for at least one of the X-axis and the Y-axis can detect a rotational angle θz of the substrate stage around the Z-axis.
Actuating an actuator (not shown) such as a linear motor based on the position information obtained from the laser interferometers can drive the substrate stage to a predetermined place.
In addition, as the NA of the lens increases with a reduction in circuit size, the allowable range of focus (depth of focus) for transferring an image on a reticle onto a substrate decreases, resulting in severer accuracy required for positioning in the focus direction (Z direction). For this reason, it is necessary to control the substrate stage upon accurate measurement of the Z-axis direction (focus direction) perpendicular to the X-Y plane, the tilt in the X-axis direction (to be referred to as rotation, tilt, or θy around the Y-axis), and the tilt in the Y-axis direction (to be referred to as rotation, tilt, or θx around the X-axis). As shown in FIG. 1, there is provided a scheme which has two X-axis laser interferometers 3A1 and 3A2 of two systems arranged side by side in the Z-axis direction to simultaneously perform position measurement, and measures the tilt θy of the substrate stage 1 in the X-axis direction from the difference between the measurement data. Likewise, arranging Y-axis laser interferometers 3B1 and 3B2 of two systems can measure the tilt θx associated with the Y-axis direction.
There is proposed a technique of measuring the position of the substrate stage 1 in the Z-axis direction also by using a laser interferometer. FIG. 2 shows an example of the arrangement of a Z laser interferometer for detecting a position in the Z-axis direction. The laser light emitted from the Z laser interferometer is reflected upward by a 45° reflecting mirror 4A mounted on the X stage. A 45° reflecting mirror 4B placed on a lens base as a reference reflects the laser light in the horizontal direction. A 45° reflecting mirror 4C placed near almost the lens center projects the laser light downward. A reflecting mirror 4D placed on the substrate stage 1 vertically reflects the laser light. The laser light then returns along the same path. The reflecting mirror 4A is mounted on the X stage. As the X position of the substrate stage 1 moves, the reflected laser light also moves in the X direction. Therefore, as the 45° reflecting mirrors 4B and 4C, bar mirrors which are elongated in the X-axis direction are used. Using such type of mirrors makes it possible to irradiate the substrate stage 1 with laser light at the same position even if the substrate stage 1 moves in the X-axis direction. When the substrate stage 1 moves in the Y-axis direction, since the 45° reflecting mirror 4A is mounted on the X stage, the position of the reflecting mirror 4A does not change, and hence the position of laser light does not change. For this reason, the relative position between the Y position of the substrate stage 1 and laser light moves, and hence a bar mirror 4D which is elongated in the Y-axis direction is placed on the substrate stage. This makes it possible to always irradiate the mirror surface of the substrate stage 1 with laser light even if the substrate stage 1 moves in the Y-axis direction. It is therefore possible to always perform measurement with the laser interferometer even if the substrate stage 1 moves in the X-Y plane.
It is also possible to measure the relative positions of the substrate stage 1 and the lens base surface by placing a 45° bar mirror 4E extending in the Y-axis direction on the substrate stage 1 and placing a bar mirror 4F extending in the X-axis direction on the lens base, as shown in FIG. 3, instead of the Z interferometer in FIG. 2.
In either of the arrangements shown in FIGS. 2 and 3, Z interferometer arrangements are symmetrically provided on the left and right sides (to be referred to as the L and R sides, respectively) of the stage. If positions in the Z-axis direction on the L and R sides can be simultaneously measured on the entire X-Y plane, the Z position of the substrate stage 1 can be measured more accurately by averaging the measurements. In addition, measuring the difference between them can also measure the tilt of the substrate stage 1.
When the substrate stage 1 is driven in the X-axis direction, the plane accuracy of the bar mirrors 4B, 4C, and 4F extending in the X-axis direction influences the positioning accuracy. Likewise, when the substrate stage 1 is driven in the Y-axis direction, the plane accuracy of the bar mirrors 4D and 4E extending in the Y direction influences the positioning accuracy. Although nm-order accuracy is required as the positioning accuracy of the substrate stage 1 in the Z-axis direction, as described above, it is technically difficult as well to process the entire surfaces of bar mirrors with nm accuracy and assemble them.
Japanese Patent Laid-Open No. 2001-15422 has proposed a technique of improving the positioning accuracy in the Z direction at all X-Y positions on a substrate stage by measuring an error originating from the process accuracy of bar mirrors in advance by using a focus sensor and setting a target position in consideration of the error when driving the substrate stage. An error originating from the process accuracy of bar mirrors is nothing but a Z error (to be referred to as a “stage moving plane” hereinafter) caused by X-Y driving of the substrate stage which is measured by using the focus sensor. This technique measures a substrate mounted on the substrate stage or a reflecting plane substituting for the substrate by using a focus sensor. Although the surface shape of a substrate influences this measurement, using a plurality of focus sensors will remove the influence of the surface shape of the substrate and measure only a stage moving plane error. As shown in FIG. 5, this technique measures a given portion P on a substrate first by using a focus sensor 6A, and then measures the same portion P on the substrate by using a focus sensor 6B after driving the substrate stage. Since the focus sensors 6A and 6B measure the same portion P, the same measurement values should be obtained regardless of the shape of the substrate. If the measurement values differ from each other, it indicates that the substrate stage has an error in the Z direction, that is, the stage is influenced by the process accuracy of the bar mirrors. This makes it possible to measure the shapes of the bar mirrors without any influences of the substrate surface.
The measurement technique for a Z bar mirror (a generic term for a ZX bar mirror and ZY bar mirror) which uses focus sensors excels in the capability of auto-calibration requiring no special tool. Not only a Z bar mirror but also an X bar mirror or Y bar mirror may deform due to an impact produced at the time of resetting (home seeking) of the apparatus or deterioration with time. It is therefore necessary to periodically measure the shape of the bar mirror. The technique of measuring the shape of a bar mirror without requiring any special tools is very important.
As shown in FIG. 4, it is also possible to use a technique of forming a stage capable of Z-tilt driving on a substrate stage 1 which slides on an X-Y plane with reference to a substrate stage base surface, and measuring the distance between the substrate stage 1 and the Z-tilt stage by a linear encoder. The difference between the technique shown in FIG. 5 and that shown in FIG. 4 is nothing but whether a measurement target is a bar mirror or a stage base surface. Although the Z-tilt measurement technique using a bar mirror will be described below, a stage base surface can also be measured in the same manner.
As described above, the Z bar mirror measurement method using the focus sensor excels in that the apparatus can perform only auto-calibration without requiring any special tools. However, this measurement device performs measurement by using a substrate, and hence has a drawback that it can measure only within the range of the substrate. In general, the range in which high positioning accuracy is required in the Z-axis direction is only the range on a substrate. For this reason, the above drawback does not pose any serious problem. For example, a reference mark is recorded on a substrate stage. Even if this mark is located outside the range of the substrate, a Z error at the position of the mark can be measured and be subjected to offset management in advance.
If, however, a mark on a substrate at a position different from an exposure position is measured by using an OAS or the like, or if focus measurement or alignment measurement is performed in a place different from the exposure position as in the case of a twin-stage arrangement, the above drawback poses the following problems.
[Problems Arising in Single-Stage Arrangement Using OAS]
As shown in FIG. 6, the OAS is spaced apart from the lens. For the sake of descriptive convenience, assume that the OAS is spaced apart from the lens by Yo [mm] in the Y direction. Since the OAS also needs to measure all the surfaces of the shots on the substrate, the ZY bar mirror must be elongated by Yo [mm]. However, a ZY bar mirror measurement device using a plurality of focus sensors can measure only a given area on a substrate. For this reason, the elongated portion corresponding to the length Yo for this OAS cannot be measured. If a plurality of focus sensors is arranged at the OAS position, the elongated portion corresponding to the length Yo for this OAS can be measured. However, since a measurement mark is very small, placing a plurality of focus sensors provides no merit in terms of cost and accuracy. Therefore, the bar mirror measurement technique using a plurality of focus sensors cannot be used at the OAS position. As described above, while the bar mirror is not properly measured at the OAS position and the stage moving plane is not corrected, driving a mark on a substrate below the OAS will produce a measurement error on the substrate stage in the Z-axis direction. For this reason, when the focus sensor performs measurement below the lens, it is necessary to greatly drive the substrate stage in the Z-axis direction, resulting in reductions in throughput and the productivity of the apparatus.
[Problems Arising in Twin-Stage Arrangement]
As shown in FIG. 7, in the twin-stage arrangement, since the OAS performs focus measurement, a focus sensor is generally placed at only the OAS position. It suffices to place at least one focus sensor below the lens to measure an error after stage switching. If the error after stage switching is sufficiently small, it is not necessary to use any focus sensor. For this reason, it is not possible to measure the Z bar mirror at the exposure position by using a plurality of focus sensors. Since the shape of the Z bar mirror cannot be measured below the lens, even if the shape of the entire substrate surface is measured at the OAS position, the substrate stage has an error in the Z-axis direction due to the influence of the Z bar mirror when exposure is performed at the lens position. This makes it impossible to accurately match the substrate surface with the lens image plane. As a result, defocus occurs, and a pattern cannot be transferred onto the substrate.